Thermal quenching is important in the heat treatment processes of metal objects. For aging hardenable materials, like many cast aluminum alloys, the thermal quenching helps to develop a supersaturated solid solution for subsequent precipitation hardening. The higher supersaturation usually leads to better mechanical properties (especially yield strength) through subsequent aging/precipitation hardening processes. The extent of the supersaturation of strengthening elements in a solid solution after quenching is strongly dependent upon the quenching rate. Rapid quenching/cooling usually results in high solute supersaturation. As a result, the material is frequently quenched into cold or warm water to maximize solution supersaturation.
Many metal parts, such as engine blocks and cylinder heads, have complex shapes and different wall thicknesses. A significant amount of residual stresses and distortion can be developed in the metal parts even when they are quenched in warm or boiling water. When tight tolerance is required for manufacturing the parts, the resulting distortion can be costly and difficult to correct. Although service performance is an important factor for reducing residual stresses, another incentive is for a reduction in distortion during machining.
One approach to reducing the difference in the cooling rate between different sections of a part is the use of a milder quenching medium such as hot/boiling water, water-polymer or polymer solutions, or even forced air quench. Although air quenching is one of the most effective ways to reduce residual stresses and distortion, it can dramatically lower the mechanical properties of the final product. FIG. 1 shows an example of a significant reduction of residual stress in a cylinder head with an air quench v. a water quench. FIG. 2 shows an example of the reduction of tensile properties with an air quench.
Therefore, there is a need for a method to improve the mechanical properties of cast aluminum components slowly quenched/cooled after solution heat treatment and/or solidification.